Touch screens user interfaces generally employ a transparent touch screen substrate disposed over a display. In operation, a user touches the transparent touch screen substrate and the touch and often the location of the touch are detected. The touch may be detected in a variety of ways—for example, by sensing a change in capacitance at the location of the touch. In one example, frustrated total internal reflection (FTIR) techniques may be used to detect the location of the touch. A touch screen employing frustrated total internal reflection generally includes an optical energy source positioned to illuminate the transparent touch screen substrate such that there is total internal reflection within the transparent touch screen substrate. That is, the light waves are reflected completely internally between the surfaces of the transparent touch screen substrate. When a user contacts the transparent touch screen substrate, however, the contact affects the index of refraction of the material, and light is scattered out of the transparent touch screen substrate in the location of the contact. Sensors are positioned to detect this scattered light, and the location of the touch can then be determined. In a similar manner, diffusive laser imaging employs laser light shined across a screen. Contact with the screen perturbs the laser light, and the location of the touch may be determined. A user device employing touch screen technology may accordingly interact with a user based on the location of a touch.
Pulse oximetry utilizes optical energy to illuminate a fingertip and determine pulse rate and blood oxygenation. Two wavelengths may be used to illuminate the fingertip, one corresponding to an absorbance maxima of oxygenated hemoglobin (typically 660 nm), and another corresponding to the absorbance maxima of deoxygenated hemoglobin. (typically 910 nm). Generally, a pulse oximeter monitors the ratio of absorbance or reflection of one wavelength relative to another and determines the blood oxygenation level of the fingertip based on the ratio. The fraction of reflected or absorbed light may also fluctuate with pulse, the pulse oximeter may simultaneously measure pulse rate.